1. Field of the Invention
This invention relates to the selective hydrogenation of butadiene contained in commercial streams containing unsaturated hydrocarbons. In particular, this invention relates to the treatment of hydrocarbon mixtures, in which butene-1 is present in a substantial amount of at least 30% and is to be preserved, with hydrogen in the presence of a solid hydrogenation catalyst, such as a palladium-containing catalyst, to convert the butadiene (1,3) to butene with avoidance of first isomerization of butene-1 to butene-2 and second hydrogenation of butene-1 and of butene-2 to butane.
The invention is also applied to the selective hydrogenation treatment of other impurities present in butenes such as acetylenes and sulfur compounds such as mercaptans and other unidentified impurities which react with hydrogen. Thus, this invention relates to a process for purifying butene-1 containing streams.
Butene-1 is of commercial interest as a modifier in the production of polyethylene and as a raw material for polybutene manufacture. Other uses include catalytic dimerization of butene-1 to octenes and 2-ethyl-hexene-1, and use as a raw material in rhodium catalyzed hydroformylation processes to produce normal pentylaldehyde. For such uses a butene-1 rich stream containing not more than 500 ppm, preferably not more than 100 ppm butadiene, is desirable as feedstock to prevent catalyst aging and byproduct polymer formation.
Butene streams obtained from cracking of hydrocarbons are treated for recovery of contained butadiene and thereafter typically contain 0.3% to 6.0% of residual butadiene which makes these cracked streams without hydrofining unsuitable as raw material for the above mentioned uses.
Since butene-1, which is an alpha olefin, is the desired starting material for the above mentioned applications rather than its non-reactive or less reactive isomers having an internal double bond, cis- and trans-butene-2, any loss of the former due to isomerization of butene-1 to butene-2 should be kept to a minimum as well as any loss due to hydrogenation to butane, during hydrofining. A useful hydrofining process would, therefore, have a threefold purpose: (1) to remove butadiene adequately by hydrogenating it to n-butene, (2) to minimize conversion of n-butene to n-butane, and (3) most importantly and most difficultly, to minimize isomerization of butene-1 to butene-2. Butene streams from steam cracking after most of the co-produced butadiene has been extracted and after isobutylene has been removed typically contain 30-75% of butene-1 and may be fractionated, if desired, to contain 75-99.5% butene-1, but such fractionation does not substantially remove the residual butadiene impurity. The hydrofining step for removal of residual butadiene can be carried out prior to or after butene-1 has been concentrated by fractionation. Isomerization of butene-1 to butene- 2 occurs at high reaction rates in the presence of hydrogenation catalysts, especially at higher temperatures and higher catalyst concentrations, because butene-2 is thermodynamically more stable than butene-1; in fact, the thermodynamic equilibrium at reaction temperature is about 90% butene-2 and less than 10% butene-1.
2. Description of the Prior Art
In U.S. Pat. No. 3,485,887, assigned to Bayer, a process is described for the hydrogenation of C.sub.4 fractions containing butene-1 and butadiene by contacting the same with a palladium catalyst wherein the C.sub.4 hydrocarbon is allowed to trickle down in the liquid phase at an inlet temperature of 10.degree. to 35.degree. C. and an outlet temperature of 60.degree. to 90.degree. C. over the fixed bed catalyst in a hydrogen atmosphere. The data show that there is very substantial isomerization of butene-1 to butene-2 and, in fact, the claims state this to be the object.
In U.S. Pat. Nos. 3,113,983, also 3,655,621, and 4,078,011, sulfided molybdenum/cobalt and nickel catalysts and higher temperatures are used to hydrogenate butadienes in C.sub.4 fractions but have the potential to introduce sulfur contamination which is a severe poison to catalysts which are used with the hydrofined butene-1 product in manufacture of modified polyethylene, octenes and oxo alcohols.
Other catalyst types have been used, viz., copper-nickel in U.S. Pat. No. 3,481,999, in which vapor phase and high temperature is used in examples, and in which it is stated that hydrogen can be a multiple of that theoretically required "for example five or tenfold molarwise". Copper plus another metal, such as Ni, Pd, is used in U.S. Pat. No. 3,076,858; and copper chromite in U.S. Pat. No. 2,964,579, both in connection with higher temperature such as 140.degree.-200.degree. C. and 150.degree.-250.degree. F., respectively. However, large concentrations of metal on the support are required in these methods. In U.S. Pat. No. 3,478,123, a ruthenium chloride catalyst is used at temperatures ranging from ambient to 300.degree. C. with hydrogen pressures of 1 to 100 atm. In U.S. Pat. No. 3,804,916 a catalyst of nickel, palladium or platinum intercalated in graphite is employed. U.S. Pat. No. 2,946,829 stresses deposition of the palladium at the surface of the support and passes a gas mixture of propylene over the catalyst to hydrogenate acetylenes and diolefins therein.
In British Pat. No. 1,497,627 a propylene cut which is to be hydroformylated with a rhodium-triphenylphosphine catalyst, is prehydrogenated in the vapor phase over a palladium-chromium, alumina supported catalyst. The patent is not concerned with treatment of a C.sub.4 fraction containing n-butenes so that the special problem of the isomerization of butene-1 to butene-2 is not considered. Propylene is not capable of isomerization. Thus, this patent does not recognize that in a commercially sized, adiabatic reactor the inlet temperature would increase across the reactor as a result of the heat of reaction in vapor phase operation and give a much higher reactor outlet temperature, especially when isomerization reactions and unwanted hydrogenation are occurring. The use of a vapor phase in this patent is limiting the ultimately achievable selectivity.
Studies have been made of the mechanism of the platinum metals as selective hydrogenation catalysts in gas phase reactions, in which the identity of the metal chosen is stressed, see Chemistry and Industry, Oct. 17, 1964, pp. 1742-1748, and see "The Hydrogenation of Alkadienes, Part. I. The Hydrogenation of Butadiene Catalyzed by the Noble Group VIII Metals", Bond, Webb, Wells and Winterbottom, Journal Chemical Society (A) 1965, pp. 3218-3227 where it is stated, p. 3225, that the routes to each butene isomer are equally affected by changes in the availability of absorbed hydrogen.
"The Hydrogenation of Alkadienes", Part II by Webb and Bates, Journal Chemical Society (A) 1968, pp. 3064-3069 discusses selectivity differences for palladium and other noble metal catalysts but also does not recognize that process conditions rather than catalyst composition can be used to achieve selective hydrofining because the authors operated in the presence of a large excess of hydrogen.
Textbook references include "Advanced Organic Chemistry" by Fieser and Fieser, Rheinhold Publishing Co., 1961, p. 181 in which the mechanism of hydrogenation on a catalyst surface is discussed for palladium and platinum catalyst, and "Hydrogenation Catalysts" by R. J. Peterson, Noyes Data Corp. 1977, pp. 199-200. In fact, it has hitherto generally been accepted in the trade and literature that selective hydrofining of butene streams to remove diolefins, acetylenes and sulfur is accompanied by the isomerization of a significant portion of butene-1 to butene-2 in the league of at least 4 to 15%, and that adequate hydrogenation of impurities is achieved by increasing the hydrogen partial pressure until sufficient conversion of impurities is attained. This increase in hydrogen pressure, it has now been found, is detrimental as it favors butene-1 to butene-2 isomerization and butene hydrogenation.